This invention relates to a method of producing a waveguide for transmitting various kinds of lights such as infrared light and far infrared light including carbon dioxide laser light.
A carbon dioxide laser has been used as a surgical laser knife in the medical field and used for welding and cutting materials in many fields of the industry. A general method of producing a waveguide for transmitting carbon dioxide laser light is described in the Prior Art section of Japanese Laid-Open (Kokai) Patent Application No. 188506/86. More specifically, a thin layer of germanium (dielectric layer) is formed on an outer peripheral surface of an aluminum pipe by sputtering, and then a nickel layer (reflecting layer) is formed on the germanium layer by plating. Thereafter, the aluminum pipe is dissolved and removed by an etchant, composed of sodium hydroxide, to provide a waveguide which is composed of the inner layer of germanium and the outer layer of nickel. The cross-sectionally circular central bore of the waveguide defined by the inner peripheral surface of the germanium layer serves as a waveguide path.
In the waveguide produced by the above conventional method, the inner peripheral surface of the nickel layer serves as a reflecting surface which reflects and transmits light, and the germanium layer cooperates with the inner peripheral surface of the nickel layer to enhance the efficiency of reflection.
Transmission characteristics of the laser depends on the smoothness of the inner peripheral surface of the nickel layer. Therefore, the inner peripheral surface of the nickel layer must be smooth. To achieve this, the outer peripheral surface of the pipe must be rendered smooth by polishing.
However, in the case of polishing the pipe of aluminum, the smoothness of the polished surface is limited, and the surface can be polished to a surface roughness of about 0.02 .mu.Rz at best, and therefore it is difficult to reduce a transmission loss satisfactorily. Also, in the case where the pipe is narrow, that is, small in diameter, there is encountered another problem that it is difficult to polish or abrade the pipe because of its reduced strength. Further, the nickel layer formed by plating is relative thick so that it can withstand a bending force or the like, and therefore the nickel layer is disadvantageous in that it is not sufficiently flexible.
In the above-mentioned Japanese Laid-Open Patent Application No. 188506/86, there is disclosed a method of producing a waveguide in which an electrically-conductive plastics tube is used as a reinforcement layer, and a metallic reflecting layer and a dielectric layer are formed on the inner peripheral surface of the plastics tube by plating. In such a waveguide, the metallic reflecting layer can be made thin since the waveguide is reinforced by the plastics tube, and therefore the waveguide is excellent in flexibility. However, in the case where the plastics tube is narrow, it is difficult to form the reflecting layer on the inner peripheral surface of the plastics tube by plating.
In order to reduce the transmission loss, waveguides having a rectangular cross-section or an oval cross-section have been proposed. In use of such a waveguide, laser light is deflected in such a manner that the direction of the electric field coincides with the longer axis of the cross-section of the central bore of the waveguide.
Japanese Laid-Open Patent Application Nos. 13412/82 and 140202/85 disclose a method of producing a waveguide in which a plurality of elongated metallic plates each having a mirror surface at its inner side are connected together by welding, by an elastic tube surrounding the metallic plates, or by fitting one metallic plate on another, to thereby provide the waveguide of a rectangular cross-section. In such a conventional waveguide, it is difficult to highly precisely position the metallic plates with respect to one another, and the manufacturing cost is rather high, and the waveguide is inferior in flexibility.
Japanese Laid-Open Patent Application No. 233705/86 discloses a waveguide comprising a metallic pipe of an oval cross-section, the inner peripheral surface of the metallic pipe serving as a reflecting surface. This conventional waveguide is also inferior in flexibility.